1. Field of the Invention
This invention relates to time-lapse free-radical polymerizable compositions and to the use of chelating agents to provide time-lapse free-radical polymerization reactions.
2. Description of the Prior Art
Numerous applications for polymers involve formation of the polymer in place. All of these applications involve the placement of a lower viscosity material on a suitable substrate followed by a controlled viscosity increase to a desired level. This increase may be brought about by solidification of a polymer melt, by evaporation of a solvent, or by a polymerization or crosslinking reaction. These polymerization and crosslinking reactions may be initiated by heat and a catalyst, or by a catalyst-activator combination which is effective at room temperature.
In many applications heat may be neither available nor practical, and thus polymerization must be conducted at ambient temperatures. For example, when the soil around a construction site is being stabilized by impregnating it with a polymerizable composition, it is not practical to apply heat to this reaction. When using polymerizable adhesive compositions to laminate large beams, the requirement of a large oven to promote polymerization of the adhesive composition would add considerably to its expense. In other cases substrate materials are heat sensitive, and thus cannot be placed in an oven. In many cases, solvent evaporation is not suitable because of insufficient control of reaction rate, because of rulings against air pollution, or because of inefficiency in confined quarters.
In German Pat. No. 848,258 a room temperature initiator system is described. This initiator system contains an oxidizing agent such as benzoyl peroxide, a reducing agent such as benzoin or acetoin, and a metal compound such as acetylacetone iron.
When an ambient temperature polymerization, initiated by a catalyst-activator system, is employed, time becomes a critical factor. Time is required to mix the monomer or prepolymer with the catalyst and to apply the polymerizable material to the desired substrate. The polymerization reaction must be slow enough to allow these operations, but it must be fast enough to give polymer formation in a reasonable length of time. As the amount of time required for applying the polymerizable material increases, by necessity, the rate of polymerization must be slowed down. Slow polymerization is especially undesirable, however, when clamps or molds are tied up in the operation or when the finished material is needed in a short period of time.
An alternative is to add an inhibitor to the polymerization system which gives an induction period. Such an inhibitor reacts with the active species of the catalyst-activator system thus stopping polymerization until the inhibitor is consumed. The length of the induction period will depend on both the catalyst and the inhibitor concentration. The larger the amount of inhibitor, the longer is the induction period and the smaller is the amount of catalyst available at the end of the induction period. Since the rate of polymerization is usually a function of the catalyst concentration, longer induction periods generally lead to a slower ultimate rate of polymerization. When long induction periods followed by a rapid polymerization are required, prohibitively high concentrations of both inhibitor and initiator are demanded.